This code initializes the options:

//Prepare the passed options
this.options = $.extend({}, options);
var o = this.options;
$.extend(o, {
	axis: o.axis || (element.offsetWidth < element.offsetHeight ? 'vertical' : 'horizontal'),
	maxValue: !isNaN(parseInt(o.maxValue,10)) ? parseInt(o.maxValue,10) :  100,
	minValue: parseInt(o.minValue,10) || 0
});

The docs say, for axis:

Normally you don't need to set this option because the plugin detects the slider orientation automatically. If the orientation is not correctly detected you can set this option to 'horizontal' or 'vertical'.

Looks to me like the specified option is always overwritten by the autodetection.